The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Radio frequency (RF) receivers typically rely on hard-switched mixers to perform down conversion. Down conversion refers to translation of an incoming modulated signal from a higher frequency to a lower frequency. Up conversion from a lower frequency to a higher frequency is also performed when transmitting the signal.
FIG. 1 shows an example of a portion of an RF receiver 10 to perform down conversion. The RF receiver 10 includes an antenna 19 that is coupled to an optional RF filter 20 and a low noise amplifier 22. An output of the amplifier 22 is coupled to a first input of a mixer 24. A second input of the mixer 24 is connected to a local oscillator (LO) 25, which provides a reference frequency. The mixer 24 converts radio frequency (RF) signals to intermediate frequency (IF) signals. An output of the mixer 24 is connected to an optional IF filter 26. The RF receiver 10 may include additional circuits to perform additional down conversion and processing.
FIG. 1B shows an example of a portion of an RF transmitter 50 to perform up conversion. The transmitter 50 includes a variable gain amplifier (VGA) 84 that receives IF signals and that is coupled to an optional IF filter 85. The optional IF filter 85 is connected to a first input of an IF to RF mixer 86. A second input of the mixer 86 is connected to an oscillator 87, which provides a reference frequency. An output of the mixer 86 is coupled to an optional RF filter 88. The optional RF filter 88 is connected to a power amplifier 89, which may include a driver. The power amplifier 89 drives an antenna 90 through an optional RF filter 91.
When a frequency range of the incoming modulated signal of the RF receiver has an upper-bound larger than twice a lower bound, harmonic mixing can be an issue. Unwanted signals lying on harmonic frequencies of the local oscillator are down converted and interfere with the wanted signal, which lies on a fundamental frequency of the local oscillator.
Harmonic rejection mixers attempt to reduce odd harmonic mixing. Intrinsic device matching is typically exploited to reject even harmonics. However, these approaches are not always sufficient to reduce even and odd harmonics, which adversely impacts silicon yield.
As used herein, η refers to duty cycle and ηnom refers to 50% duty cycle. DCoffset refers to DC offset with respect to a predetermined threshold voltage or current value (such as zero). Balanced η refers to duty cycle equal to 50% (or ½) and unbalanced η refers to duty cycle not equal to 50% (or ½). IMn refers to nth order inter-modulation. As IMn decreases, linearity increases. IIPn refers to nth order input referred intercept point. IIP is proportional to 1/IMn. Therefore, linearity increases as IIPn increases.
FIG. 2 illustrates a duty cycle of a square wave signal. The pulse width τplus represents a period that the square wave signal is positive while the pulse period T represents a period of the local oscillator square wave. T is related to the fundamental frequency fLO of the LO as fLO=1/T.
Duty cycle is typically defined as the ratio of the pulse width to the period T and it is quantified as a percentage. For example, if the pulse width to period ratio is ½, then the effective duty cycle is 50%. Under these conditions, the square wave LO spectrum includes only odd harmonics of the fundamental frequency as can be seen in FIG. 3. Moreover, these harmonics have energy than decreases with increasing frequency. When the effective duty cycle is not equal to 50%, the LO spectrum also includes even harmonics as can be seen in FIG. 4.
FIGS. 5-7 illustrate a hard-switching mixer performing down-conversion in a receiver. Incoming RF signal energy at fLO±fIF is converted to a lower frequency fIF. The operation produced by the hard-switched mixer can be viewed as a multiplication, in the time domain, between the incoming RF signal and the sign of the local oscillator waveform. The time domain multiplication with the sign(LO) is what differentiates hard-switched mixers from analog multipliers where the RF incoming signal is multiplied with the LO waveform.
Because of the periodic and bipolar nature of the LO waveform, sign(LO) is a square-wave with the same period as the original LO waveform. Consequently, if the effective duty cycle of sign(LO) is 50%, the spectrum includes the odd harmonics. If the incoming RF signal includes only energy around the fundamental frequency of the LO (fLO), the rich harmonic content of sign(LO) is not an issue since only the useful/wanted energy will be translated around fIF and processed. However if the incoming RF signal has energy around the harmonics of the LO, hard-switched operation may create issues.
Referring now to FIGS. 8-9, when the incoming RF signal energy spreads close to any of the LO harmonics with some interferers, the nature of the hard-switched mixer downconverts the interferer to IF as well. Harmonic mixing distortion occurs when the interferer energy folds over the useful signal, which corrupts the information in the useful signal. Harmonic mixing distortion is an issue in wireless systems that handle RF bands (incoming for the RX and outgoing for the TX) where the upper bound is 2 or more times the lower bound.
When the LO is balanced (or equivalently has a duty cycle of 50%), harmonic distortion should be considered for odd LO harmonics. However, when the LO has a duty cycle that is different than 50%, even order harmonics appear in the sign(LO) spectrum. RF interferers located close to the even LO harmonics fold over the useful signal. Dotted lines in FIG. 9 illustrates the case where the LO has a duty cycle different than 50%, which leads to the down conversion at fIF of the interferer placed at fIF+2fLO.